Why do our eyelashes and eyebrows never continue to grow unlike hair on our head?

 Answer 1: The cilia or eyelashes are hairs situated on the margin of the lids. They are dispensed in two rows, totaling about 100-150 cilia in the ipper lid and half the number in the lower lid. The pigmentation of the cilia is deeper than that of the scalp hair throughout adult life.The average life of each cilium is from 3-5 months after which it falls out and a new one grows in to take its place. If the cilia is pulled out the new one replacing it reaches full size in about 2 months. Since the life of the cilia is only 3-5 months, it does not grow as long as scalp hairs.

  

Answer 2 : Small organs in the skin, called follicles produce hair. All follicles go through a three-stage cycle- anagen is a period when the hair grows actively; catagen is marked by a short phase during which growth winds down; and finally telogen is the stage of rest. The cycle is completed at the end of telogen and the hair falls and a new hair begins to grow.Scalp hairs (hairs on the head) have the longest anagen period - four to eight years during which they grow and have just two to four months of rest stage. Eyelashes, arm hair, and most other hairs on the body have significantly shorter growth periods of one to six months, followed by a 2-4 month period of rest. That is why these hairs are shorter and appear to grow to a fixed length. Thus the length of time that the hair is able to spend growing during the growth phase controls the maximum length of the hair. The Hindu